Modeled car disappears in 'play mode' of game engine
Your normals are facing the wrong direction. The game engine does not render faces which are facing away from the camera since these are behind the object. However, your normals are facing the wrong way which causes the exact opposite to happen - back faces are drawn, front faces are not.


Select the problematic object and switch to edit mode with Tab.
Press A to select everything.
Press W-R to remove any duplicate vertices (as they may break the next step).
Press Ctrl-N to make normals consistent.